The present invention is directed to a process and apparatus for the production of directly pressable (moldable) uranium dioxide powder from uranium peroxide (UO.sub.4.xH.sub.2 O) which is continuously precipitated from uranyl nitrate solution whereby by variation of the precipitation conditions as well as the temperature in the subsequent calcination and reduction the properties of the UO.sub.2 powder are so regulated that in the pressing and sintering of tablets (pellets) it is possible to vary the sintered density in the large range of 75-95% of the theoretical density without additional conditioning steps such as plasticizing, granulating or sieving.
In order to take steps against the swelling of the fuel at the higher burn-up rates in modern light water reactors, the sintered density in the UO.sub.2 pellets has been reduced from 95% of the theoretical density partially up to 90%. Such a variation of the sintered density is not possible with the previously known processes for the production of uranium dioxide and of the thus obtained powders. The future reactor generation, the sodium cooled breeder reactor, however, must contain pellets with a sintered density of only 80-85% of the theoretical density. For this purpose, the presently known processes for the production of uranium dioxide powder must employ a conditioning of the UO.sub.2 powders. Either the entire UO.sub.2 powder must be subsequently calcined at 1100.degree.-1400.degree. C. or be mixed with a large portion of highly calcined UO.sub.2. Such powders, however, are inclined partially to resintering which cannot be permitted for the operation of the reactor since the fuel elements are damaged thereby.
There have become known a series of precipitation processes according to which sinterable UO.sub.2 is produced from uranyl nitrate solutions. However, there are only a few processes, as, e.g., via the precipitation of ammonium uranyl carbonate, with which it is possible to produce a free-flowing UO.sub.2 powder which permits a direct pressing of UO.sub.2 pellets without pretreatment. With all known processes, however, there is formed a UO.sub.2 powder from which there can only be produced pellets with relatively high sintered density.
Therefore, it was the problem of the present invention to find a process and apparatus for the production of a free-flowing uranium dioxide powder which could be directly compacted and worked to pellets with an adjustable sintered density of 75-95% of the theoretical density without granulating or other aftertreatment.